Visual masking by transcranial magnetic stimulation in the first 80 milliseconds

Stimulation of the occipital cortex with transcranial magnetic stimulation (TMS) can interfere with visual processing and may cause masking comparable to visual masking. The effect is most pronounced when the TMS pulse is delivered with stimulus onset asynchronies (SOAs) of 80-100 ms. In a few experiments a second time window of TMS-induced visual masking has been identified with its maximum around an SOA of 40 ms. The existence of two masking windows has been taken as evidence for two distinct visual processes taking place in V1: an early feedforward component and a later re-entrant feedback component. The evidence for the existence of two separate TMS time windows is reviewed. The early time window was not reproducible in all the attempts to characterize TMS masking effects. Interindividual anatomical differences in the location of V1 might contribute to the heterogeneous results.